1. Field of the Invention
A method for making particle board having low edge thickness swelling utilizing an adhesive, such as a phenol formaldehyde (PF) resin, is disclosed. The method avoids emissions of ammonia and produces low NOx emissions by tailoring the adhesive used to the process conditions. Apparatus is also provided such that the application of the resin to lignocellulosic particles is effected in a particular sequence which enables the production of building boards having low edge thickness swelling with no ammonia emissions and low NOx emissions. The resulting lignocellulosic particle boards have novel and unexpected properties.
2. Description of the Related Art
The manufacture of building board of particles adhered by an adhesive such as those obtained by PF binders is a well established art, as described, for example, in U.S. Pat. No. 1,358,394 to Redman et al issued in 1920 (incorporated by reference in its entirety). Therein is described the method of producing a phenolic condensation product by combining a phenol substance, such as phenol, with an active methylene substance, such as formaldehyde, and after mixing with a filler, may be molded in hot presses.
PF resins had become the adhesives of choice for manufacturing durable heat- and moisture-resistant wood based composites. They are low in cost and provide the high strengths required for structural applications.
However, as organic solvents became unacceptable in the building panel production processes, due to health, environmental, and flammability considerations, phenolic wood binders were provided as either aqueous resoles or spray-dried resole powders. The powder form is limited in its ability to provide properties because a secondary binder must be applied to cause the PF powder to adhere to a wood surface until the pressing step. Generally, the secondary binders are capable of retaining only about 3 wt % PF powder on the surface of the wood particles. While this amount may be sufficient for many commodity panels, it is often insufficient for developing the high strength or low moisture responses required of high value speciality applications.
On the other hand, liquid resins, such as aqueous resoles can be applied at much higher levels. At these higher-levels, they are much- more capable than powders for developing the high strengths and low moisture responses required of the speciality panel products. Though more flexible than powders, the aqueous resoles are also limited in their abilities due to the effects of the additional moisture that is carried into the system, since as the amount of aqueous resin increases, so does the weight of the aqueous vehicles for the resin. This added moisture slows the cure of the resin and may inhibit development of full cross-linking, thereby adversely diminishing the strength of the adhesive. The moisture also softens the wood substrate reducing pressure between mating wood surfaces. Furthermore, as presses heated above the boiling point of water are commonly used in the board industry to increase production speeds by accelerating the cure of the binder, the existence of additional moisture ( greater than 12%) may create high internal steam pressures during the hot pressing, leading to blows and sub-optimal adhesive contributions due to resin migration in response to steam flow. In addition, the heat applied in prior art processes increased the emission of noxious gases, such as ammonia, and if the plant is provided with a Regenative Thermal Oxidizer (RTO), the ammonia may be converted to NOX. If the plant does not have an RTO, or some other heat system that puts resin emissions through a burner, there will be no NOx formed, although in that case ammonia would be emitted to the atmosphere. Thus, although the increased addition of resin via an aqueous vehicle may lead to better properties, the process is inherently limited as a result of the concurrent moisture additions in the form of the aqueous vehicle for the resin.
The lignocellulosic particles commonly utilized in many panel producing method, such as those for making oriented strand board (OSB), typically used in sub-flooring, roof sheathing, siding and wall sheathing, along with other specialty applications, already contains moisture inherently, or introduced during preliminary processing steps.
For example, when logs of wood enter the manufacturing facility, they are typically placed in a vat or xe2x80x9chot pondxe2x80x9d to-help thaw the wood and/or remove dirt and grit from the logs before debarking the same. Alternatively, the logs may be retained in an outside storage lot before being brought into the manufacturing facility for flaking. Each of these techniques introduces moisture into the logs.
Debarked logs are xe2x80x9cflakedxe2x80x9d in flakers to provide flakes having certain properties, such as specific length, width and thickness. This results in xe2x80x9cgreen flakes.xe2x80x9d Green flakes are undried and typically have moisture contents between 20 and 80 wt % moisture content on an xe2x80x9coven-driedxe2x80x9d basis, i.e., the weight of the flakes after oven drying.
Typically, the green flakes are stored in a xe2x80x9cgreen binxe2x80x9d or xe2x80x9cwet binxe2x80x9d before drying to prespecified specified manufacturing moisture content. The green flakes thereafter are sent to driers to dry the flakes to a typical moisture content of about 2 to about 10 wt %. Dried flakes are stored in xe2x80x9cdry binsxe2x80x9d or xe2x80x9cdry flake binsxe2x80x9d until blended.
Blending is where adhesive (binders), catalyst, water and wax (emulsion or slack) are typically added to the dried flakes. Such binders are typically PF resole resin or pMDI. PF resin binders are typically applied at rates between 1.7%-8.0% (based on a wt % of solid binder to oven-dry wood). The blended flakes are transferred to forming bins, which are used to meter the flakes onto a forming surface, such as a forming belt. The forming bins contain xe2x80x9corienter rolls or discsxe2x80x9d which orient the flakes in either the direction of the forming line or transverse to the direction of forming line travel. The forming bins also control the limit of the amount of flakes falling onto the forming surface, which controls the finished panel density, which is usually between 36 and 50 pounds per cubic foot.
The forming surface travels under forming heads creating a continuous mat of oriented flakes. These mats are typically cut to specific lengths and loaded onto a xe2x80x9cpre-loaderxe2x80x9d or loading cagexe2x80x9d which is a staging area for a full xe2x80x9cpress-loadxe2x80x9d of mats.
The mats are pressed to specific thickness and the resin cured to result in a finished panel. The conditions of elevated temperature, pressure, and time can be varied to control the cure time. Catalyst can also be introduced during the processing steps to optimize the pressing times or to shorten the overall pressing time.
The finished panels are thereafter usually cut to size, stacked, painted and packaged for delivery to the customer.
Attempts have been made to reduce press time by preheating the flakes on the forming surface, such as disclosed in U.S. Pat. Nos. 5,643,376 and 5,733,396 to Gerhardt et al (incorporated by reference in their entirety). Therein, a particle mat is heated by concurrently passing through the mat treatment air coming from an air-conditioning system and having a predetermined moisture content and dew point such that the mat is preheated to a predetermined temperature while liquid in the treatment air is allowed to condense in the mat to, at most, a maximum liquid content. Other attempts to preheat the mat employs the use of microwaves; See, U.S. Pat. No. 5,913,990 to Kramer, or steam; See, U.S. Pat. No. 5,993,709 to Bonomo, or hot-air; See, U.S. Pat. No. 6,054,081 to Bielfeldt, prior to the pressing step (all patents are herein incorporated by reference in their entirety).
Other attempts for introducing an adhesive into green flakes can be found in Canadian Patent 1,135,610, issued in 1982. Processes for introducing adhesive into green flakes, as well as into the same flakes after drying, was disclosed in Canadian Patent 989,289, issued in 1976.
However, none of these processes teach a method of introducing an adhesive, in the form of either a powder or an aqueous form, to the xe2x80x9cgreen flakes,xe2x80x9d i.e., before the flakes are initially dried to a predetermined moisture content, with no ammonia and low NOx emissions occurring during one of the subsequent drying and/or pressing steps.
A method is provided for introducing a powder or aqueous resin adhesive, preferably a PF resole binder, to green flakes before they are dried, with little or no formaldehyde or ammonia emissions and low NOx emissions.
The invention also provides an apparatus suitable for forming building panels, particularly OSB panels from green particles by means of a hot press wherein at least a part of the resin binder is introduced, in aqueous or powder form, prior to the drying of the green particles.
Building panels having high strength and low edge swells and formed of lignocellulosic particles and a PF adhesive, having low NOx emissions and little or no ammonia or formaldehyde emissions when dried and/or pressed under heat and pressure are provided.